The present invention relates to an input device for a computer, and more particularly to an infrared (IR) touch screen for a video monitor that serves as a computer input device.
Infrared touch screens are known which utilize vertical and horizontal arrays of IR emitters and sensors disposed about the rim of the screen of a video monitor to form a grid of IR beams superposed in a plane over the screen of the monitor. The emitters are sequentially activated and opposing sensors are likewise scanned for sensing the presence or absence of an IR beams from the respective opposing emitters. When a pointed object or finger is placed near the screen of the monitor, thereby blocking vertical and horizontal IR beams, the position of the object in the grid is detected by the output of the X-Y sensors that are optically aligned with the blocked beams.
In known IR touch screens, the IR emitters and sensors and the hardware associated therewith for driving the emitters and scanning the sensors are built into the video monitor or retrofitted internally into the monitor. This requires either specially manufacturing a touch screen monitor, which is obviously expensive for both the manufacturer and consumer, or opening up a monitor to retrofit the touch screen optoelectronics which poses the danger of damaging internal components of the monitor, again an undesirable consequence for the consumer/user. It therefore would be desirable to have a relatively inexpensive IR touch screen device that could be externally adapted to a wide variety of video monitors for serving as an input device to a computer by simply plugging the device into the serial input port of the computer, similar to known mouse or joy stick input devices. Such an externally adaptable touch screen device could be massed produced relatively inexpensively and selectively applied to a user's monitor without risking damage to internal components of the monitor.
Further, IR touch screens that have been developed so far suffer from a relatively low resolution and slow response time. Generally speaking, overall resolution of an IR touch screen is limited by the physical size of the IR emitter-sensor pair devices. The smallest known devices of this type force a limit to the grid dimension on the order of 0.25 inches. Even if smaller emitter-sensor pairs were available to reduce the grid dimensions and thereby increase the resolution of the touch screen, the required overall scanning time would be undesirably increased due to the increased number of sensing elements that would need to be scanned and due to the fact that the scanning time is fixed for each element. It would therefore be advantageous if the resolution of an IR touch screen could be increased without decreasing the scanning speed, and if possible, to increase the resolution simultaneously with an increase in scanning speed.
Additionally, in known IR touch screens, there exists an aberrational error known as parallax due to the curvature of the screen and the normal human arm and finger mechanics, causing IR beams to the side or above or below the intended pointing location to be blocked, thereby designating an incorrect location on the screen. This problem is more pronounced at the corners of the touch screen. It would be desirable to devise a touch screen to minimize the parallax problem.
A further problem with known IR touch screens has to do with signal to noise ratio of the IR sensors which are sensitive to broad band light. The signal to noise ratio can be degraded by the variable ambient light conditions in which the touch screens may be operated. A related problem concerns the fact that the infrared emitters emit infrared light which is projected over a relatively wide angle so that the infrared light actually illuminates a number of oppositely disposed infrared sensors. Similarly, the infrared sensors have a relatively wide viewing angle and thus can sense the infrared light projected by a number of oppositely disposed infrared emitters. The output of a sensor is thus unintentionally raised by infrared light from emitters on either side of the optically aligned emitter which, combined with a low signal to noise ratio could, result in an erroneous output.